1. Field of the Invention
The present invention relates to thermosetting polycarbodiimide copolymers, a process for producing the polycarbodiimide copolymers, thermosetting resin compositions containing the polycarbodiimide copolymers, and heat-resistant flexible films, and more particularly to thermosetting polycarbodiimide copolymers that are suitably used in applications of various electronic parts, for example, as materials of base films or cover-lay films for flexible wiring boards, or adhesive films, and can exhibit not only a high heat resistance and a good flexibility, but also an excellent flexing resistance (resistance to 180° bending); thermosetting resin compositions containing the polycarbodiimide copolymers; a process for efficiently producing the polycarbodiimide copolymers; and heat-resistant flexible films produced from the polycarbodiimide copolymers or the thermosetting resin compositions.
2. Description of the Prior Art
In general, as resins having a high heat resistance, there are known thermosetting aromatic polycarbodiimides produced from raw monomers such as diphenylmethane diisocyanate and tolylene diisocyanate. Such aromatic polycarbodiimides have been used as flame-resistant films or heat-resistant adhesives because of an excellent heat resistance thereof.
The aromatic polycarbodiimide films have such an advantage that they are free from production of volatilized gases or decomposed monomer products even when exposed to a temperature of 400° C. or higher. However, the aromatic polycarbodiimides tend to suffer from self-crosslinking when heat-treated at a temperature as high as about 200° C. for a long period of time, so that films produced therefrom tend to be deteriorated in flexibility. In addition, the aromatic polycarbodiimide films themselves have a high elastic modulus and are, therefore, unusable in applications requiring bending, for example, in applications of various electronic parts such as base films or cover-lay films for flexible wiring boards, or adhesive films.
As the thermosetting resin compositions having a good flexibility and molded articles produced therefrom, there are disclosed, for example, (1) thermosetting resin compositions containing a diene elastomer-modified epoxy compound, a polycarbonate-based resin and a curing agent as effective ingredients (e.g., refer to Japanese Patent Application Laid-Open No. Heisei 5(1993)-70667), (2) thermosetting resin compositions containing (A) at least one thermosetting resins selected from the group consisting of epoxy resins, cyanate resins and bismaleimide resins, (B) a specific thermoplastic resin, and (C) a specific rubber (e.g., refer to Japanese Patent Application Laid-Open No. Heisei 7(1995)-149952), (3) thermosetting adhesive sheets containing an imide resin prepolymer, an epoxy resin and an elastomer (e.g., refer to Japanese Patent Application Laid-Open No. Heisei 6(1994)-329998), or the like.
However, the thermosetting resin compositions (1) and (2) tend to be insufficient in heat resistance owing to adverse influence of the low-heat-resistant thermoplastic resin or elastomer (rubber) contained therein, thereby failing to meet a high heat-resisting level required for lead-free soldering. Also, the thermosetting adhesive sheets (3) have an excellent heat resistance, but are difficult to handle by ordinary apparatuses since they must be heat-treated upon curing thereof at a temperature as high as 200° C. Therefore, the thermosetting adhesive sheets can be used only in limited applications. In addition, any of the above compositions and sheets contains an epoxy resin-cured component and, therefore, fails to show a good flexibility capable of withstanding 180° bending.
On the other hand, as compositions containing polycarbodiimide or a carbodiimide compound, there are disclosed, for example, (4) elastomer compositions containing a polyester, vulcanized rubber fine particles and polycarbodiimide (e.g., refer to Japanese Patent Application Laid-Open No. Heisei 5(1993)-295243), (5) fiber-reinforced polyurethane resin compositions containing a thermoplastic polyurethane resin, reinforcing fibers and a carbodiimide compound (e.g., refer to Japanese Patent Application Laid-Open No. 2003-201349), or the like.
All of these conventional compositions are thermoplastic resin compositions which are enhanced in high-temperature characteristics by blending a small amount of polycarbodiimide or the carbodiimide compound therein and, therefore, tend to be still unsatisfactory to meet a high heat-resisting level requiring for lead-free soldering.
In addition, there are disclosed polycarbodiimide copolymers containing a polyfunctional liquid rubber component in a molecule thereof (e.g., refer to Japanese Patent Application Laid-Open No. Heisei 11(1999)-322888). The polycarbodiimide copolymers have been developed for the purposes of improving a thermoforming property as well as controlling a swelling property under heating. Thus, the conventional materials still fail to satisfy not only a high heat resistance and a good flexibility, but also a flexing resistance (resistance to 180° bending).
Further, there are disclosed carbodiimide copolymers containing hexamethylene carbonate diol in a molecule thereof (e.g., refer to Japanese Patent Application Laid-Open No. 2002-279830). The carbodiimide copolymers are resins capable of being softened upon heating, but fail to show a good elasticity, i.e., a sufficient flexing resistance owing to crystallinity of the hexamethylene carbonate component contained therein.